I don't think there is a fixed way.
There are some girls well known and open example Adrianna formly Pure 90% she will stick you in raw if not you can ask without worries.
Some require additional things such as drugs, alcohol or money to consider.
Some you need to be a regular before that option opens up, but of course in some cases it's all included and other cases they will want something see above.
Some ask straight up if it's offered can be risky as times this may anger one of them, but some won't have a problem you asking, so it can be a little risky.
Other times it's either catching certain hints they may give sometimes directly and sometimes indirectly. 5 years ago I saw a girl now retired after our first session she said maybe we can get tested and have sex raw. The next time I saw her I went in raw without us getting tested but that was hint enough.
I usually ask a girl to do pussy slide and sometimes that can help, if a girl really is pushing boundaries a good chance she might be ok with it, sometimes they may just stick it in other times they may giggle and maybe you can ask lol
Also some girls maybe attracted to some guys more and therefore it may open menu more.